Soda cook of acid sulfite knotter rejects



United States Patent This invention pertains to a new and useful process for treating acid sulfite stock and acid sulfite knotter rejects. More particularly, this invention pertains to a process for the separation of a selected wood fraction from said stock and knotter rejects followed by soda cooking of said fraction to obtain pulp fiber therefrom.

In the preparation of paper by the well-known acid sulfite pulping process, a certain amount of the cooked material discharged from the digester is undesirable and must be removed from the pulp stock before it can be made into commercially acceptable paper products. The amount of such material will vary widely and includes knots, uncooked and partially cooked wood chips, pieces of bark, dirt and bark carried by the wood, and fiber bundles or shives. Their separation is normally accomplished on the basis of particle size, although there is increasing use of the centrifugal principle which separates particles on the basis of density. Usually, in normal acid sulfite pulping processes, the stock from the blowpits or washers is diverted to a screen, an example of which would be a Ionsson Knotter, with /4" diameter drain openings for sepa ration of the large particles of undesirable materials from the pulp fibers. These undesirable materials are generally termed acid sulfite knotter rejects.

In accomplishing the separation described above, there is some loss of good fibers along with the knotter rejects. To minimize the loss of good fiber it is conventional practice to further subject the knotter rejects to another Jonsson Knotter screen 0A" diameter openings) where additional fiber is gleaned from the rejects before they are sent to the boiler house for burning to recover their fuel content. Included in the rejects which are sent to the boiler house are knots, pieces of bark, uncooked and partially cooked good wood chips, shives and fiber bundles. If the fiber bundles and good wood fraction could be separated from knotter rejects and thus be rendered substantially free of the most undesirable constituents such as large knots and large pieces of bark, substantial economic benefits would result, for the value of the paper that could be produced from the recovered fiber bundles and good wood fraction exceeds the value of the rejects as fuel.

As indicated in the above discussion, the expression acid sulfite knotter rejects as used herein and in the claims, includes all types of undesirable materials that must be removed from the stock during the acid sulfite pulping process in order to obtain pulp fibers that can be used in making commercially acceptable articles. This material may also sometimes be referred to as tailings or simply rejects. It includes undesirable material contained in acid sulfite pulp stock which may be removed by, for example, knotters, fiat screens, vibratory screens, rotary screens, as well as by centrifugal means. It comprises, for example, knots, bark, uncooked and partially cooked chips, fiber aggregates, shive-forming constituents, and dirt and bark carried by the wood.

Other methods or processes have been practiced for preparing pulp from the aforesaid undesirable materials or knotter rejects, but none of the prior methods was able to achieve the preparation of high quality pulp in an economical manner, as is possible with the process of this invention. One such prior method, which is typical, is to refine the total acid sulfite knotter rejects, followed by several cleaning steps. Such a method, however, yields a pulp "ice having a high bleach demand requiring excessive bleaching chemicals, and often produces a dirty pulp, thus making the recovery of pulp from knotter rejects unattractive from the standpoints" ofquality and economics. Another method has been toa-di'vert the total knotter rejects to digesters in which they are subjected to normal kraft processing. Again, however, such a method yields a pulp having a high bleach requirement, further increases the consumption of chemicals and processing time in the kraft process, and increases the dirt level in the final pulp above normal quality specifications.

A principal object of this invention, therefore, is to provide a process for preparing pulp from acid sulfite knotter rejects which is economical to operate, yet yields a high quality pulp.

Another object of this invention is to provide a process for preparing pulp from acid sulfite knotter rejects which has a low bleach requirement and can be bleached in combination with normal acid sulfite mill pulp without causing undue brightness or shive and dirt problems in the pulp mixture.

Another object of this invention is to provide a process for preparing a high quality pulp from acid sulfite knotter rejects which requires a surprisingly low consumption of chemicals and is simple to operate.

Additional objects and advantages will be apparent as the invention is hereinafter described in detail.

According to the present inventon, to obtain the fraction acceptable for soda cooking, a special screen of arelatively large mesh is designed to remove the most undesirable constituents (knots in particular) is used, followed by screening of the thus obtained accepted fraction by a screen of a substantially smaller mesh to separate fibers from the fraction to be subjected to soda cooking.

A preferred embodiment of a process according to this invention involves the steps of: (a) screening acid sulfite knotter rejects to remove the most undesirable constituents (large knots, pieces of bark, etc); (b) screening the accepted fraction which passes through the screen of step (a) to remove pulp fibers; and (c) soda cooking the fraction retained on the screen of step (b) to obtain pulp.

The foregoing procedure results in a pulp which is of low bleachability and high quality. These results, surprisingly, are attained with a much lower caustic consumption than is used on fresh wood chips by normal alkaline processes. In fact, it is possible to operate such a process in an economically beneficial manner without recovery of the cooking chemicals.

In another embodiment of the invention, acid sulfite pulp stock comprising knots, pieces of bark, uncooked and partially cooked chips and fibers, as received from the blowpits or washers of a normal acid sulfite process, is passed to a screen to remove large knots, pieces of bark and other impurities and the accepted fraction is sent to a second screening step wherein the pulp fibers pass through the screen and on to normal processing and the fraction retained on the second screen is forwarded to a digester where it is subjected to soda cooking to obtain pulp therefrom.

The following detailed description is intended to be illustrative only, and should not be regarded as unduly limiting the invention.

In the first preferred embodiment of the invention which was set forth in a general manner hereinabove, acid sulfite knotter rejects are obtained in the usual way from a normal acid sulfite pulping process involving either a softwood or hardwood or both, the rejects being diluted to a consistency, e.g. about one to two percent, that is suitable for screening. The diluted knotter rejects are passed to a screen having a relatively large mesh designed to give the best separation of the most undesirable constituents such as large knots, pieces of bark and wood fragments from the accepted or desirable fraction, which is composed mainly of good wood chips and fiber bundles. The accepted wood fraction is further screened by a screen having a substantially smaller mesh to'obtain the fraction for soda cooking. The percentage of undesirable constituents removed by this step is not uncommonly in the neighborhood of about 25 percent.

The screen used in this step which may be called the prescreening step may be of a vibratory, rotary or flat type. Preferably, a vibratory screen is employed, and it is advantageous to position it in piggyback fashion over the screen used in screening the accepted fraction. Alternatively, the screen for the prescreening step can be positioned over a belt conveyer or the like, which will receive the accepted fraction and deliver it to the second screening step. The prescreening mesh size was designed for optimum removal of knots and other undesirableconstituents, after a careful study of the constituents of the whole knotter reject fraction. A screen mesh of about /2" by about 1%" was found to give excellent separation of the most undesirable constituents from the good wood fraction, composed mainly of wood chips and fibers and giving a yield of about 75 percent. This is believed to be the optimum size opening and yield for the particular facilities in which the invention has been tested. However, it is contemplated that any screen with generally rectangular openings of about to about A" by about to about 3" may be employed.

The exact optimum size and shape of the screen opening may vary somewhat from mill to mill and should in each case be selected to give optimum removal of the most undesirable constituents from the smaller sized good wood or accepted fraction.

The material which passes through or is accepted by the above-described pre-screening is next introduced to a second screening device. The purpose of this screening is to separate pulp fibers from the accepted fraction obtained in the prescreening step. The screen used here can be the one normally used for separating pulp fiber from the undesirable constituents in a conventional acid sulfite process and may be, for example, a Jonsson Knotter screen with openings of A" diameter. The fibers removed at this point are diverted to normal processing in the mill screen room, whereas the rejected fraction retained on the top of the screen is sent to, for example, a dewatering trough fitted with a screw conveyor, for transport to digesters for soda cooking. The digesters for soda cooking may be of the conventional type used in a normal alkaline cooking process.

As an alternative to the foregoing described procedure of screening the knotter rejects obtained from a normal acid sulfite pulping process, one may insert the screen used in the above described prescreening step ahead of the point where the screening operation of a normal acid sulfite pulping process usually occurs. Thus, in this embodiment, the entire acid sulfite pulp stock is passed to the relatively large mesh screen where the most undesirable constituents are retained by the screen and the accepted fraction composed of the pulp fibers and wood fraction is passed through the screen. The accepted fraction is then passed to a second screen of substantially smaller mesh to separate the pulp fibers from the Wood fraction. This second screen is the screen to which the pulp stock is normally first subjected after cooking in the normal acid sulfite pulping process. The pulp fibers are processed through normal processing channels and the rejected wood fraction which was retained in the top of the second screen is sent to the digesters for soda cooking and the obtaining of pulp fibers therefrom.

Other alternative embodiments embracing the concept of this invention will be obvious to those skilled in the art. For example, in a conventional process, the acid sulfite stock washed in the blow pits is pumped to Jonsson knotter screens GA diameter openings) for removal of the knotter rejects. The Jonsson knotter screens are sometimes overloaded and, as a consequence, a loss of good fiber along with the rejects occurs. Knotter rejects from two or more screens are usually combined andsent to another Jonsson knotter screen where additional fiber is gleaned from the rejects before their diversion to the boiler house. By means of the concept of this invention, the foregoing problem is overcome by installing a relatively large mesh screen as described heretofore between the primary knotter screening and secondary knotter screening used for recovery of good fiber. The large knots, pieces of bark, etc. retained by the relatively large mesh screen go to the boiler house, the fiber recovered by the secondary screening goes back into the process, and the rejected wood fraction retained by the second screening is diverted to a digester for soda cooking. The secondary knotter screen also acts as a dewatering device, which is desirable, since it is well to remove as much entrained water as possible from the rejected fraction prior to introducing it to a digester for soda cooking.

Soda cooking of the rejected fraction obtained by any of the above-described embodiments is accomplished in a conventional manner by use of digesters such as used in a normal alkaline cooking process. The cooking time and temperature is variable. Temperatures below 120 C. are rather impractical because of the long cooking time. Cooking temperatures of 120 C. would correspond to cooking times of about 6 hours to about 10.5 hours. Cooking pressure is a function of liquor to wood ratio and liquor strength. Liquor to wood ratios canbe varied, 'but a minimum ratio is the most desirable from an economic standpoint.

The following examples serve to illustrate the soda cooking step of the invention and should not be regarded as unduly limiting the invention. Unless otherwise indicated, all parts are expressed as parts by Weight. Screened yield is the ratio of the weight of air dry pulp Bone Dry by definition) which was produced after cooking and subsequently screening on a 0.012" by 0.014" fiat screen, to the initial Bone Dry (B.D.) rejected wood fraction fed to the digester. The ratio of liquor to B.D. wood is the ratio of the weight of liquor in the digester, including the water and dilute caustic soda, to the B.D. weight of the rejected food fraction fed to the digester.

Example 1 The rejected wood fraction of acid sulfite knotter re jects, prepared according to the screening steps described hereinabove, was charged into a digester. The chips used were from hemlock, white fir and lodge pole pine species. The ratio of sodium hydroxide to B.D. wood was 0.16 and the ratio of liquor to B.D. wood was 5.8.

The charge was cooked for 3 hours at a cooking temperature of about 165 C. and at a maximum pressure of p.s.i.g. The digester was then emptied through a blow tank and the stock screened and washed in the ordinary manner. The final screened yield was 40.1 percent. The stock bad a bleach demand of 8.3 percent chlorine.

Example 2 In this run, the ratio of NaOH to B.D. wood was 0.15, and the ratio of liquor to B.D. wood was 4.5. The charge was cooked for 1.5 hours at a cooking temperature of 174 C. and at a maximum pressure of 110 p.s.i.g. This run resulted in a screened yield of 49.3 percent. The stock had a bleach demand of 7.4 percent chlorine.

Example 4 In this run, the ratio of NaOH to B.D. wood was 0.15 and the ratio of liquor to BB. wood was 4.5. The charge was cooked for 3.5 hours at a cooking temperature of 171 C. and at a maximum pressure of 122 p.s.i.g. This run resulted in a screened yield of 44.9 percent. The stock had a bleach demand of 4.7 percent chlorine.

ExampleS The rejected fraction of the knotter rejects removed by the prescreening step was subjected to essentially the same conditions as those used in Example 4. This run resulted in a screened yield of 35.2%. The stock had a bleach demand of 133+ percent. The percent screening was 26.9 which compares to less than one percent on the pulp made from the accepted wood fraction of the second screening step.

As is evident from the foregoing examples, the production of low bleachability pulp at good yields with a caustic consumption which is much lower than that used in normal alkaline pulping processes is accomplished by the process of this invention. Caustic consumptions in normal alkaline processes are usually the equivalent of 30 percent of the Bone Dry wood. Cooking temperatures may vary a great deal, and it has been found advantageous to operate between about 160 to about 180 C., which will result in a caustic consumption of 15 percent or less of the Bone Dry wood. The ability of the process of this invention to utilize much lower caustic concentrations to produce a high quality pulp permits the sewering of blow liquid, which is not economically possible in normal alkaline pulping processes.

The strength of the pulp prepared .by the process of this invention is slightly weaker than that of normal pulp. The pulp of this invention can be blended in with normal pulp at a concentration of, for example, 1%, in which case there is no detectable ditference in the strength characteristics of the paper made from the mixture and that made from normal pulp alone.

Pulp prepared by the process of this invention may be bleached by itself by conventional bleaching methods. Alternatively, the pulp can be blended with normal mill unbleached pulp and then bleached together in a normal bleaching sequence.

What is claimed is:

1. A process for treating acid sulfite knotter rejects which comprises the steps of:

(a) separating said knotter rejects into an accepted fraction characterized by material which will pass through a screen of relatively large mesh and a fraction characterized by material which will not pass through said screen;

(b) separating said accepted fraction into a rejected fraction characterized by material which will not pass through a screen having a mesh substantially smaller than the screen of step (a) and a fraction characterized b'p material which will pass through said screen having a smaller mesh; and

(c) subjecting said rejected 'fraction to soda cooking to obtain pulp therefrom.

2. A process for treating acid sulfite knotter rejects which comprises:

(a) separating said knotter rejects to obtain an accepted fraction characterized by material which will pass through a screen having openings of about /8 to about 21" 'by about A" to about 3", and a fraction characterized by material :which will not pass through such a screen;

(b) separating said accepted fraction to obtain a rejected traction characterized by material which will not pass through a screen having about A" diameter openings and a fraction characterized by material which will pass through such a screen, and;

(c) subjecting said rejected fraction to soda cooking to obtain pulp therefrom.

3. A process for treating acid sulfite pulp stock comprising knots, pieces of bark, uncooked and partially cooked chips and fiber bundles which comprises:

(a) separating said stock to obtain an accepted fraction characterized by material which will pass through a screen having generally rectangular openings of about /2" by about 1% and a 'fraction characterized by material which will not pass through said screen;

(b) separating said accepted fraction to obtain a rejected fraction characterized by material which will not pass through a screen having about A" diameter openings and a fraction characterized by material which will pass through said screen having about A" openings; and

(c) subjecting said rejected fraction to soda cooking to obtain pulp therefrom.

References Cited UNITED STATES PATENTS 731,290 6/1903 Drewsen 162-55 1,729,629 10/1929 Richter 16255 3,046,184 7/1962 Braun 16286 DONALL H. SYLVESTER, Primary Examiner.

H. R. CAINE. Examiner. 

1. A PROCESS FOR TREATING ACID SULFATE KNOTTER REJECTS WHICH COMPRISES THE STEPS OF: (A) SEPARTING SAID KNOTTER REJECTS INTO AN ACCEPTED FRACTION CHARACTERIZED BY MATERIAL WHICH WILL PASS THROUGH A SCREEN OF REALTIVELY LARGE MESH AND A FRACTION CHARACTERIZED BY MATERIAL WHICH WILL NOT PASS THROUGH SAID SCREEN; (B) SEPARATING SAID ACCEPTED FRACTION INTO A REJECTED FRACTION CHARACTERIZED BY MATERIAL WHICH WILL NOT PASS THROUGH A SCREEN HAVING A MESH SUBSTANTIALLY SMALLER THAN THE SCREEN OF STEP (A) AND A FRACTION CHARACTERIZED BP MATERIAL WHICH WILL PASS THROUGH SAID SCREEN HAVING A SMALLER MESH; AND (C) SUBJECTING SAID REJECTED FRACTION TO SODA COOKING TO OBTAIN PULP THEREFROM. 